Acrylic acid, an α, β-unsaturated carboxylic acid, is an important commodity chemical. When reacted with an alcohol it forms the corresponding ester. Acrylic acid and its esters readily combine with themselves or other monomers by reacting at their double bond to form homopolymers or copolymers useful in the manufacture of various plastics, coatings, adhesives, elastomers, floor polishes and paints.
Traditionally, acrylic acid is derived from fossil hydrocarbon resources. The most widely used process for acrylic acid manufacturing is the vapor phase oxidation of propylene, which is a byproduct of ethylene and gasoline production, involving two reactions in series, using two separate catalysts. The other method for acrylic acid manufacturing involves hydroxycarboxylation of acetylene. This method utilizes nickel carbonyl and high pressure carbon monoxide, both of which are expensive and considered environmentally unfriendly. In addition, there is a concern in the continued use of fossil hydrocarbon reserves in the manufacture of acrylic acid as it contributes to an increase in the greenhouse gas emission. As a result, there is a growing interest in the catalytic dehydration of lactic acid and 3-hydroxypropionic acid as an alternative route to produce acrylic acid because lactic acid and 3-hydroxypropionic acid can be derived from renewable, biological resources like sugar cane, corn and cellulosic feedstock.
A number of inorganic solid acid catalysts have been reported to be useful in the production of acrylic acid from lactic acid at elevated temperature. The production of acrylic acid from lactic acid involves removal of hydroxyl group from alpha carbon atom and hydrogen atom from the adjacent beta carbon atom. Thus, it would appear that the efficiency of this chemical conversion from lactic acid to acrylic acid would depend on the rate constant for the dehydration reaction. But in reality, the challenge in increasing the efficiency of dehydration of lactic acid leading to acrylic acid production depends on inhibiting a number of competing side reactions. As illustrated in FIG. 1, under the conditions reported to be favorable for dehydration of lactic acid, four other competing chemical reactions namely decarbonylation, decarboxylation, condensation and reduction are known to occur either in parallel or in series. Acetaldehyde formation occurs when lactic acid undergoes decarboxylation or decarbonylation reaction. Condensation reaction involving lactic acid at elevated temperature results in the formation of 2,3-pentanedione. Reduction reaction involving lactic acid at elevated temperature results in the formation of propionic acid and 1,2-propanediol. Thus in a catalytic reaction involving solid acid catalysts at elevated temperatures, lactic acid yields a product mixture comprising acrylic acid, acetaldehyde, hydroxy acetone, 2,3-Pentanedione, propionic acid and 1,2-propanediol. Fractional distillation process may be followed to separate acrylic acid from the resulting product mixture. However, the process step involving fractional distillation adds additional cost to acrylic acid manufacturing process. Therefore, it is desirable to develop a catalytic process for manufacturing acrylic acid involving catalytic dehydration of lactic acid where the formation of byproducts such as 2,3-pentanedione, propionic acid, hydroxy acetone and acetaldehyde are either completely eliminated or significantly reduced.